Filter devices of this type can be arranged for example in industrial facilities in a hydraulic circuit, in order to filter impurities out from the hydraulic medium. The hydraulic medium can be a lubricating oil or a coolant. Preferably it is a coolant/lubricant. Lubricants which serve at the same time for cooling can be used for example in processing machines. In such applications, this may concern an open hydraulic circuit in which at least a portion of the coolant/lubricant is introduced directly into the processing zone. In the case of a processing by machining, an intensive contamination of the coolant/lubricant occurs. Such impurities are removed from the coolant/lubricant by means of filter devices which are presented here. The purified coolant/lubricant can then be used again.
By the filtering operation, filter elements which separate a raw side from a clean side within the respective filter device can gradually become clogged by the separated impurities. A so-called filter cake forms on the raw side on the filter element. Through these accretions, the through-flow resistance of the filter element increases. At the same time also a significant improvement to the filtration effect occurs.
In order to avoid damage to the filter element or respectively the filter device in the case of too high a pressure rise on the filter element, it is basically possible to provide a bypass for circumventing the filter element, which can be controlled by means of a bypass valve. The bypass valve then opens in the case of an unacceptably high differential pressure between the raw side and the clean side.
A filter device is known from DE 101 05 612 A1, in which a filter element separates a raw side and a clean side in a filter housing, and in which a bypass valve controls a bypass circumventing the filter element. For this, the bypass valve has a valve member for controlling a bypass opening, which connects fluidically with the clean side a coupling chamber of the bypass valve which is fluidically connected with the raw side. The valve member is driven into a closed position, closing the bypass opening, by a pretension produced by a closing spring. The pretension acting on the valve member can be changed by means of an actuating device. In the known filter device, the pretension can be set in a stationary manner to a desired opening pressure by means of the actuating device, depending on the respective purpose of use of the filter device, before the installation of the filter device.
Further filter devices in which the opening pressure, at which a bypass valve opens, can be set in a stationary manner for the respective case of application, are known from DE 20 2007 013 216 U1 and from DE 14 36 289 A.
From FR 15 75 877 A1 a filter device is known which comprises a valve arrangement with a congestion indicator device. The congestion indicator device here has an indicator element which is connected with a membrane mounted between the raw side and the clean side. A congestion of the filter element leads to a pressure rise on the raw side, which leads to a corresponding adjustment movement of the membrane and hence to a corresponding adjustment movement of the indicator element.
In hydraulic systems, in particular in a coolant/lubricant circuit, different system pressures can come into use. For example, a load or working operation and an idle operation can be differentiated from each other. In an idle operation, for example, a minimum volume flow can be necessary, in order to be able to maintain a lubrication of the machines which are connected to the hydraulic circuit. In order to consume as little energy as possible here, a comparatively low pressure level prevails in the hydraulic system during such an idle operation. In order to be able to guarantee the low pressure of the idle operation also in the case of a contaminated filter element, the bypass valve must be able to open already at a relative low pressure. During a working operation or load operation, the hydraulic system must also be able to realize the desired cooling in addition to the lubrication. Accordingly, a greater volume flow is necessary. In addition, in the working operation, the contamination of the coolant/lubricant occurs, so that the full filtration effect of the filter element is to be exhausted. Accordingly, it is desirable to use the system pressure which is available for the filtration of the entire volume flow. Consequently, the bypass valve must only open for the working operation at a relatively high opening pressure. Therefore, contradictory requirements arise for the bypass valve. In addition to this there are particular operating conditions, such as for example a cold start of the hydraulic system, in which the respectively used hydraulic medium, in particular an oil, has a higher viscosity than at its operating temperature.